Nissan Patrol 4x4 (Africa)

I'll try summarise.

Had some increasing electric/electronic issues, discovered parasitic drain of around 3 amps, only on the main 120 amp fuse, not at any of the other fuses. I assumed it was the alternator, and the alternator was noisy anyway, so bought a replacement.
Then I was sidelined so didn't fit the alternator or continue troubleshooting.

Fast forward a few weeks and I fit the new alternator and discover I still have the parasitic drain and it is now up to 5amps.

Where must I continue looking for the problem?

The last time I drove it it went into limp mode as I was leaving the house. Limp mode cleared when I fired up again but it makes me want to get the car running 99% ok before risking driving it to a sparky to give it a once over

Forgive the ignorance but is that 120amp fuse connected to the main engine wiring harness? As in does it only protect the main engine circuitry? If it does I would just replace the whole harness if you are struggling to find the cause of the drain. I had to do mine last year as the shielding on some of the wires was brittle and cracked and was causing faults and limp mode sagas. New one here in bots cost me P3800 the labor was another P3000 anyway the point is that the guys said it would be nearly impossible to trace the exact point on the exact wires that were causing the issue and if they had to do it the labor charge would be worse and they would probably cause further damage to the harness in the process. Might just be easier to cut your losses and replace the harness. There are 3 engine wiring harnesses, the main one for P3800 and then 2 secondary ones for P1200 respectively. In nissans own words I quote "nissan wiring is not their strongest point and as the 4.8 is a naturally hot running engine the under bonnet temps eventually cause the shielding on the wires to go brittle and crack".

I might be completely off the field here but that's my experience in any case good luck, I hope you get it sorted without too much fuss

5A is a lot!

I would start by pulling all the relays one at a time while measuring, and if that doesn't work, maybe try and move wiring around while measuring, and see if you can track it down that way?

Replacing the whole harness is a great way to sort it, but rather expensive too...

LOBO did the same with me -- Sparky took days to establish it is the RADIO
So I just remove the face and end of drain .. I must still find the cause as it did not do it before

The 120A fuse is the alternator circuit and only has one off-take that runs to the ignition switch and runs the alternator light. Not a lot that can cause drain in that circuit. Unless that wire was used accidentally as a + ignition wire and an accessory is drawing power on it.

I suspect the problem is faulty rectifier on the alternator. Although it is a new one so very strange if it was that. To test put a multi meter set to AC V over the battery terminals with the engine running. Any sign of an AC current means bad diodes.